The present invention relates to optical detectors and more particularly to a novel shutter assembly for an optical detector head.
Known optical detector heads contain a sensitive detector element, such as a photocell and the like, for furnishing an electrical signal in the event a radiation of a predetermined wavelength, such as ultra-violet radiations emitted by a flame and the like, impinge thereon. The photocell is typically housed in an orientable container or detector head which serves to cool the photocell, to focus the incident radiation upon the photocell, and carefully aim the photocell at a particular zone of the radiation source, such as a flame, to be monitored. It is well known that the efficiency and selectivity of an optical detector increases as the photocell is brought closer to the source of radiation. In particular, when an optical flame detection head is used with a flame-producing burner of the aiming type, the detector head must be capable of proper angular movement to follow the aiming movement of the burner. In certain cases, detector heads extend directly into the interior of a boiler or furnace to place the photocell in close proximity to the flame.
Any photocell, especially one sensitive to ultra-violet radiation, is subject to breakdown whereby the photocell is unable to distinguish between the presence or the absence of the desired optical wavelength radiation. It is therefore advisable to frequently verify the operational efficiency of the photocell. Verification is normally accomplished by means of an optical shutter which interrupts the radiation path between the source to the photocell. During the obscuration period, the condition of disappearance of the signal furnished by the photocell is utilized to verify the integrity of the photocell.
The ambient conditions associated with the use of photocells inside a detector head subject to the above-described conditions results in frequent mechanical malfunctions. For an optical flame detector, the mean and maximum environmental temperatures are very high, the utilized materials tend to change as a result of the irradiation, and soot frequently infiltrates into the mechanism. The optical shutters previously employed in the art normally use levers, rods, gears and the like to obtain shutter movement and require a relatively large amount of preventive maintainance as the shutters are particularly subject to breakdowns. Because of these disadvantages, such shutter mechanisms are not compatible with the desired uninterrupted service of the installation as a whole unit. Additionally, the optical shutters known to the art generally do not insure total obscuration of the flame-emitted radiation due to the necessity to employ rather coarse mechanical tolerances as required to overcome the collateral effects of temperature and soot infiltration.
Such disadvantages become greater when the movable parts of the shutter have unbalanced movement masses. Many known shutters cannot be used in all positions as their movement is often counteracted by the weight of the movable parts themselves. This disadvantage requires that a greater motor force be used to overcome the larger resistance mechanical wear which increases the amount of friction between the parts until the shutter device soon becomes inefficient and unusable. Frequently, those devices previously known to the art could only function in certain positions.